Some winter seasons seem to produce an apparent
surge in incidences of ceiling/floor partition
separation problems. The theories to explain this
increase vary from the weather (a mild but wet
winter), to lumber drying practices (a shift from
KD-15 to KD-19 production), and to a changing
lumber resource (more juvenile growth wood).
Whether any of these theories have any truth to
them is anyone’s guess.

One thing that is bound to drive up reports of
problems is awareness. The phenomenon known as
CFPS or Ceiling/Floor Partition Separation has
probably been around since the time that using
large amounts of insulation in attics became
common. Articles on the problem started showing
up in the 1970s and many more were written in the
1980s. Today, many builders are not only aware of
the problem if it occurs in their area, but they also
know a little about how and why it happens. Just
note how quickly and reliably that a case of CFPS is
blamed on the trusses. But many cases are not
caused by truss arching at all; or else truss movement
is only one part of the problem.

This leaves the wood truss industry in the
position of solving someone else’s problem. Even
if trusses are the main problem in a case of CFPS,
most of the repair options are things that could have
been done by the builder in the first place to avoid
the problems. Since the design and manufacture of
wood trusses cannot control the occurrence of
CFPS, what should the truss industry’s response be
when someone claims that trusses are arching up?
This article will look at what truss suppliers,
builders and designers can say and do when drywall
joints crack and gaps open up.

What can be said?

Natural shrinking and swelling of wood

Separations between interior partition walls and
the ceiling or floor can occur for a variety of
reasons. One of the causes is known as truss
arching or truss uplift. It is the natural shrinking and swelling of the wood that results in this movement.
The movement can repeatedly cycle up and down
in response to seasonal changes in the wood’s
moisture content.

Not a structural problem

Truss uplift movement does not indicate a
structural problem or a lack of strength in the wood truss. Building owners can be assured that the
structure will not fail. There is nothing that the
truss manufacturer or truss designer can do to a
truss to prevent CFPS.

Do not fix the truss

There is nothing that can be done to the truss to
permanently and reliably eliminate the movement
without reducing the structural integrity of the
truss. Cutting truss members to force a truss to
move down will weaken the roof or floor structure.
It can also redistribute the reaction forces to walls
that are not bearing walls and cause other problems.

Float the drywall joints

The real problem is that the drywall attachment
and the connection between trusses and partition
walls are not constructed to allow for expansion
and contraction of the structure. Proper detailing of
these connections so that the drywall joints will
“float,” is the best way to avoid the problem and
also to fix the problem once it has happened. In
other words, the fix is to do what could have been
done in the first place to avoid the problem.
What can be done?

Truss suppliers

As a truss supplier, you can document that you
have performed your duty to inform the users of
your product about potential problems. An information
sheet can be included with the delivery of
trusses and you can require the builder to sign a
receipt acknowledgement. A sample of such an
information sheet is shown on the following pages
of this issue of "Good Connections". You may wish
to state that you would not be responsible for any
drywall or other finish problems if floated drywall
corner construction is not used.

Building designers

As a building designer, you can specify floated
drywall corner joints and appropriate connections
between trusses and partition walls. There are
several framing hardware products available that
make these details and connections simple to do.

Builders

As a builder, use framing practices that are
compatible with trusses. Generally, this means
making only a light connection between trusses and
non-load bearing partition walls. You can also
provide ceiling drywall nailers that permit the
ceiling boards to be floated at the partitions. And, of
course, insist that the drywall contractor use floated
corner construction.

The summer season is not the time of year when
the truss uplift phenomenon is given much attention
since seasonally occurring gaps tend to close up
during warm weather. But next winter’s problems
are being built now. One of the difficulties is that
most builders rarely experience a case of CFPS. It is
hard for them to justify the cost of preventive
measures. Fixing an occasional problem is cheaper
than reducing the possibility of an occasional
problem. This is a risk that the builder may prefer to
take. From the truss supplier’s point of view that is
all right as long as the builder will take responsibility for the consequences.

IMPORTANT INFORMATION FOR CONTRACTORS & BUILDERS
ABOUT CEILING/FLOOR PARTITION SEPARATION

Ceiling/partition separation has occurred
occasionally in wood framed buildings causing
gaps in the drywall along the corners between the
ceiling and the partition walls. Ceiling/partition
separation is caused by a combination of material
characteristics and environmental conditions, and
can be prevented by using correct and well established
construction practices.

Causes of ceiling/partition separation

Bearing and support settlement.

Shrinkage of framing lumber, joists, and
headers due to drying of lumber.

Raising of the roof trusses.

Excessive deflection of floor below the
partition.

Bearing and support settlement and shrinkage of
framing lumber can cause gaps that will not reoccur
after the structure has settled and reached an
equilibrium level.

Excessive deflection of the floor below the
partition can be caused by inadequate distribution
of the partition load to the supporting structural
members or inadequate load carrying capacity.
Gaps due to this condition will initially increase,
but level out after the initial deflection and creep in
the structural material has taken place.

Raising of the roof trusses can cause seasonal
appearance of gaps. They open up in winter and
close again in summer. The up and down movement
of the truss is caused by differential shrinkage
and expansion of the chord members of the roof trusses. This is caused by changes in the moisture content of the wood.

Encasement of bottom chords in thick layers of
insulation, improper ventilation and a larger
percentage of juvenile wood in the construction
lumber are factors that contribute to the movement
of the trusses. The truss configuration, production
techniques, or other variables within the control of
truss designers or truss manufacturers are not the
cause for this phenomenon.

Recommendations to avoid ceiling/partition separation

Make sure that foundations are of the right
size and properly placed and on suitable bearing
material. Request an inspection by a professional
engineer if in doubt.

Provide adequate structural supports under
all partitions.

Use dry lumber for headers and load bearing
structural framing. Prevent exposure of these
structural components to rain for long periods.

Protect trusses from exposure to rain during
extended periods of storage.

Do not ventilate kitchen and utility vents
into the attic space.

Provide adequate ventilation from the attic
space to the outside.

Insist that the drywall contractor use a
floating corner construction. The ceiling drywall
must be secured to the wall with a metal clip or
some other suitable means to prevent it from
rising up with the trusses (see recommended application and finishing of gypsum board).

Studies of ceiling/partition separation problems
have shown that one or several of the above shown
items have been ignored. Following the above
recommendations will not guarantee that there will
never be a problem, but they will help to eliminate
the effects of the presently known causes for
ceiling/partition separation. Useful publications
for proper installation of trusses and correct
ventilation are "Handling, Installation, and
Bracing Metal Plate Connected Wood Trusses
HIB-91" from the Truss Plate Institute and "Principles
of Attic Ventilation" by AirVent, Inc.

Recommended application and finishing of gypsum board

Alpine’s recommendation on how to attach
gypsum board to wood trusses to avoid cracks or
open joints at wall/ceiling lines. Please read
carefully!

Floating interior angles

To minimize the possibility of fastener popping
in areas adjacent to the wall and ceiling intersection,
the floating angle method of application may
be used for either single or double layer application
of gypsum board to wood framing. This method is applicable where single nailing, double nailing or
screw attachment is used. Gypsum board should be
applied to ceilings first. (See figures) Floating
interior angles should be used where fire ratings are
required.

The first attachment into each ceiling framing
member framed perpendicular to the intersection
should be located 12 inches out from the wall
intersection for ½” board and 16 inches for 5/8”
board.

Gypsum board on sidewalls should be applied so
as to provide a firm support for the floated edges of
the ceiling gypsum board. The top attachment into
each stud should be located 8 inches down from the
ceiling intersection for single nailing or screw
applications. (See figures) Also, see Gypsum
Association GA216-85.

David B. Brakeman, P.E., S.E., is Alpine’s vice president of engineering. He joined Lumbermate Company as a design engineer in 1978 and became a member of the Alpine Engineered Products family when Lumbermate was acquired in 1989. He holds a bachelors degree in physics from Lawrence University and a bachelor of science degree in civil engineering from Washington University in St. Louis, Missouri, where he serves as a part-time lecturer.